This application is based on and incorporates herein by reference Japanese Patent Application No. 2000-137633 filed on May 10, 2000.
1. Field of the Invention
The present invention relates to a direct current motor, a yoke housing of the direct current motor, and a manufacturing method of the yoke housing.
2. Description of Related Art
In a direct current motor, when an electric current is applied to armature coils of the motor, a magnetic flux is generated due to an armature magnetomotive force. This magnetic flux influences a magnetic flux of permanent magnets (primary magnetic poles) and causes a phenomenon of so called xe2x80x9carmature reactionxe2x80x9d. When the armature reaction is excessively large, a spatial magnetic flux distribution is greatly distorted, causing displacement of an electrically neutral axis in the motor. Thus, a magnetic flux density in a commutating zone becomes far apart from zero, inducing many problems, such as generation of sparks, a reduction in an output power of the motor due to a reduction in a magnetic flux of the primary magnetic poles, or the like.
One way of solving the above-described disadvantages is disclosed, for example, in Japanese Unexamined Utility Model Publication No. 57-139283.
A yoke housing of a motor disclosed in this publication has notches that penetrate through a wall of the yoke housing at predetermined positions. The notches are formed such that notches provide a large magnetic resistance only to a magnetic flux induced by an armature magnetomotive force without influencing a magnetic flux of magnets (primary magnetic poles). Thus, by providing such a notch, only the magnetic flux induced by the armature magnetomotive force is reduced without influencing the magnetic flux of the primary magnetic poles, so that the armature reaction is restrained.
Furthermore, in the motor disclosed in this publication, in order to prevent intrusion of foreign matter, such as water, dust or the like, into an interior of the yoke housing through the notches, an arrowhead-shaped non-magnetic member is plugged into each notch of the yoke housing, or alternatively a cylindrical non-magnetic frame is secured around the yoke housing to surrounds the entire yoke housing including the notches.
In the motor disclosed in the above publication, the arrowhead-shaped non-magnetic member, the cylindrical non-magnetic frame member, or the like is required to prevent the intrusion of the foreign matter into the interior of the yoke housing through the notches. Thus, a cost of such a non-magnetic member is added, and an assembling step for assembling such a non-magnetic member is also added. Thus, disadvantageously, this results in an increase in a cost of the yoke housing, and thereby a cost of the motor.
The present invention addresses the above described disadvantages. Thus, it is an objective of the present invention to provide a yoke housing of a direct current motor capable of restraining armature reaction without exerting a substantial influence on a magnetic flux of primary magnetic poles and also capable of preventing intrusion of foreign matter into an interior of the yoke housing without necessitating a specialized separate member. It is another objective of the present invention to provide a direct current motor having such a yoke housing. It is a further objective of the present invention to provide a manufacturing method of such a yoke housing.
To achieve the objectives of the present invention, there is provided a direct current motor comprising a yoke housing and an armature rotatably received within the yoke housing. The yoke housing has a plurality of primary magnetic poles secured to an inner peripheral surface of the yoke housing for providing magnetic fields to the armature. The yoke housing also includes a plurality of groove-like reduced thickness portions for increasing a magnetic resistance. The plurality of groove-like reduced thickness portions are integrally formed in the yoke housing. A number of the plurality of groove-like reduced thickness portions is equal to a number of the plurality of primary magnetic poles. Each groove-like reduced thickness portion extends along a center line of a corresponding one of the plurality of primary magnetic poles.
There is also provided a method for manufacturing the yoke housing of the direct current motor. The method includes forming the plurality of groove-like reduced thickness portions in the yoke housing by pressing a die along a surface of the yoke housing in a direction axially parallel to the yoke housing, the die being also used for forming at least a portion of a rest of the yoke housing.